my buddys 500efi with a 160 tstat runs 180 to 195 all the time what is the correct t stat for the 500 efi

160* tstats for EFI's is correct.

My 502mpi's with 160* tstats always run 170-180* on the guages. I had a guage that always used to show high readings, up to 210* sometimes. I knew it wasn't reading right since the alarm should go off by 200*. I would suspect that your friend has a guage going bad.

Those engines should have a 140 from stock, but they always ran rich and if you put a 160 in they run a lot better and a little faster too no joke. Merc wanted them to run rich for engine safty

They take either a 140 or 160 it depends on the year. I have been this route. You will have to take your serial number with you to get the correct one.

Well, I just looked at www.mercruiserparts.com

Every batch of serial numbers for the 500EFI list a 140* Tstat.
Thats news to me, because I thought all the efi engines used a 160* Tstat.

575'SCi's use the 143* too.

My gauges will read between 150-160

Yes from what I remember all the HP EFI stuff run 140 and stock EFI run 160. They did it beouse they wanted more fuel to be safe and did not want them to run lean at all. I remember every one complaned the transom was black all the time and it was hard on the spark plugs too, So on some boats "not all boats" we tryed the 160 and it was a big differents. And from what I remember we saw no engine damage, "BUT THAT WAS RUN AT YOUR OWEN RISK"

So your saying his hp 500 efi should have a 140

Original/stock on all 500EFI's is a 140* according to www.mercruiserparts.com

Yes it should have a 140.